1. Field of the Invention
The present invention relates to a landing correction apparatus for preventing a possible degradation of chromatic purity caused by an influence of a geomagnetism in a color television receiver.
2. Description of the Prior Art
Generally in a color television receiver in which a display image is formed on a display screen by scanning phosphor elements through deflection of an electron beam, a landing position of an electron beam is varied by an influence of a geomagnetism to result in a change in luminance and chromaticity of the image picture displayed on the display screen. Particularly in a cathode ray tube for use in a high vision system where high density phosphor elements having a fine pitch is adopted, there occurs a greater degree of a landing error of an electron beam (referred to as "mislanding" hereinafter) due to the influence of the geomagnetism.
One such system for beam mislanding correction is shown in Japanese Patent Publication laid open 4-109785 and its corresponding U.S. Pat. No. 5,298,985 entitled IMAGE CORRECTION APPARATUS FOR ADJUSTING IMAGES BY DIGITALLY CONTROLLING ANALOG CORRECTION WAVEFORMS, issued to Tsujihara, which are incorporated herein by reference for their teachings on beam mislanding correction.
The degree of electron beam mislanding depends on the arrangement of the phosphor elements. With regard to the arrangement of the phosphor elements, for instance, when comparing a stripe type arrangement as shown in FIG. 9 (a) with a dot type arrangement as shown in FIG. 9 (b), the stripe type arrangement has a greater tolerance for the beam landing because no substantial change takes place in luminance efficiency of the phosphor element when the beam landing position varies in the lengthwise direction of the stripe phosphor element. In FIGS. 9(a) and 9(b), reference numerals 301 and 302 denote phosphor elements, and reference numerals 303 and 304 depicted by broken lines denote mislanding positions of electron beams.
The mislanding of electron beams is corrected using a landing correction coil through which an appropriate electric current is flowed for canceling the influence of the geomagnetism. As a conventional electron beam landing correction apparatus, there has been employed, for example, a beam landing correction apparatus for a color cathode ray tube as disclosed in the Japanese Patent (Unexamined) Laid-Open Publication No. HEI 2-29187.
The above mentioned conventional landing correction apparatus is briefly explained with reference to FIGS. 10 through 14.
Referring to FIGS. 10 and 12, the apparatus comprises a cathode ray tube (CRT) 101 having a display panel 101a, a deflection yoke 102, periphery landing correction coils C1 through C6 for correcting the mislanding of electron beams in peripheral portions of the screen, photosensors Ps1 through Ps6 for detecting the mislanding, a gun center coil 107 for correcting the mislanding of the electron beams at the center portion of the screen, and a measurement coil 108 for generating an auxiliary deflection magnetic field for measuring the mislanding amount. The apparatus further comprises a sample/hold circuit (S/H) 109 for sampling and holding an output of each photosensor, a multiplexer 110, an analog-to-digital (A/D) converter 111, a digital calculation circuit 112, a measurement control circuit 113, a demultiplexer 114, a latch 115, a digital-to-analog (D/A) converter 116, a drive circuit 117 for driving the landing correction coils, and a video circuit 118 for processing a video signal.
FIG. 11 shows an arrangement of the periphery landing correction coils C1 through C6 for correcting the mislanding of the electron beams at the peripheral portions of the screen.
FIGS. 12 (a) and 12 (b) show an arrangement of photosensors Ps1 through Ps6 for detecting the luminance on the screen. In FIG. 12 (a), the positions of the photosensors Ps1 through Ps6 corresponds to the positions of the landing correction coils C1 through C6 at the peripheral portions of the screen. The photosensors are covered by a frame member 137. A phosphor element 139 is provided on the inner surface of the display panel 101a at the face plate of the cathode ray tube 101.
The following describes the operation of the conventional landing correction apparatus having the above-mentioned construction with reference to FIGS. 13 and 14. FIGS. 13 (a) and 13 (b) are charts for explaining a mislanding detection operation, while FIG. 14 shows a relationship between the beam landing position and the light quantity detected by a photosensor.
First, regarding a measurement current i which is applied to the mislanding amount measurement coil 108 from the measurement control circuit 113, the current i increases in steps from a negative potential region to a positive potential region for measuring the mislanding amount in each field as shown in FIG. 13 (a). In the above case, a control signal e is supplied from the measurement control circuit 113 to the video circuit 118 so that the video circuit 118 provides, for example, a video signal for generating a green or white raster to the cathode 101c of the cathode ray tube 101.
According to change of the current flowing through the measurement coil 108, the deflection center in the deflection yoke position is changed as indicated by the beam loci in FIG. 13 (b). That is, the beam is applied to the position on the screen from a cathode 101c passing through an aperture mask 146 and the beam landing position is moved in the horizontal direction of the screen as indicated by arrows in FIG. 13 (b) (vertical direction in the figure). Reference numeral 143 denotes a center beam when no deflection is applied, and numeral 144 denotes an electron beam deflected by the geomagnetism, and numeral 145 denotes a mislanding-corrected beam deflected by the gun center coil 107.
Light quantity detection outputs of the photosensors Ps1 through Ps6 are sampled every field in the sample/hold circuit 109 as shown in FIG. 13 (a), and the sampled data is transmitted via the multiplexer 110 to the analog-to-digital converter 111 and then supplied to the digital calculation circuit 112 in a form of a digital signal.
The detected light quantity output of the phosphors takes its maximum value at the optimum landing position and gradually decrease as it departs from the optimum landing position as shown in FIG. 14. Since the mislanding amount differs in different portions of the screen, the optimum field number at which the maximum light quantity can be obtained differs in each photosensor.
The digital calculation circuit 112 calculates measurement currents i.sub.1, i.sub.2, . . . , i.sub.6 corresponding to the position of each of the photosensors Ps1 through Ps6 at which the maximum output is obtained. Furthermore, an average or a weighted average of the current values are calculated to obtain a correction current d.sub.0 to be applied to the gun center coil 107. The correction current d.sub.0 is a direct current which is used for the landing correction in a center area of the screen.
Meanwhile, correction currents d.sub.1, d.sub.2, . . . , d.sub.6 are calculated from the measurement currents i.sub.1, i.sub.2, . . . , i.sub.6. The correction currents d.sub.1, d.sub.2, . . . , d.sub.6 calculated in the calculation circuit 112 are each output via the demultiplexer 114 to the latch circuit 115 and further converted into an analog form by means of the digital-to-analog (D/A) converter 116 to be subsequently supplied via the drive circuit 117 to each of the periphery landing correction coils C1 through C6. The above-mentioned correction currents d.sub.1, d.sub.2, . . . , d.sub.6 are direct currents to be supplied to the correction coils at the peripheral portions of the cathode ray tube to correct the beam mislanding which cannot be removed by the gun center coil 107.
However, in the above-mentioned landing correction coil arrangement as in the conventional example, although the strength of the magnetic field formed by the correction coils can be controlled, the direction of the landing correction magnetic field formed by the correction coils can not be controlled, and therefore the influence of the geomagnetism directed in a variety of directions cannot be canceled, resulting in involving a problem that the beam mislanding cannot be corrected with high accuracy.
Furthermore, the optimum landing position is specified to the point at which the light quantity detected by the photosensor is at its maximum value without taking any matter of chromaticity into consideration, and therefore there also arises another problem that no sufficient uniformity in chromaticity can be achieved in the displayed picture even when a landing correction is effected.